A number of techniques for eliminating the human element in determining whether a ball or other airborne missile or projectile landed in bounds or out of bounds on a playing service have heretofore been proposed. One of these employs the mark a ball leaves upon impact with the surface of a clay court or an artificial claylike court made from a composition such as Hardtru as an indication of whether or not the ball landed out of bounds.
This technique is of course inapplicable to hard surface courts. Furthermore, if the ball contacts the surface at a small angle, the mark left in the court surface by the ball may well lie outside the boundary line, even though it brushed the boundary line in flight. Thus, the mark indicates that the ball landed out of bounds although it was officially in bounds according to the rules of tennis.
Related approaches involve coating the playing surface and/or ball with chemical compositions capable of reacting to produce a visual mark when the ball contacts the playing surface. One system of this character is disclosed in U.S. Pat. No. 4,109,911 issued Aug. 29, 1978, to John A. Van Auken for GAMING SURFACE CONTACT DETECTING SYSTEMS.
The approach just discussed has the same disadvantage as that utilizing scuff marks in the surface of a clay or artificial clay court to ascertain where a ball touched down. That is, the ball may actually contact the outer edge of the playing surface boundary in flight, and therefore be officially in, yet leave a mark which lies entirely beyond the boundary line and therefore suggests that the ball landed out of bounds.
In fact, the just-related situation is typically aggravated because the tapes employed to mark the boundaries of clay, Hardtru, and other claylike tennis courts commonly protrude slightly above the playing surface. As a consequence, a ball indicated by a visual mark to have landed as much as one-half inch out of bounds may in fact have touched the tape as it approached touchdown and therefore have officially been in bounds.
Chemically based systems also have the disadvantage that it is at best difficult to so control the chemical reactions producing the colored mark that the mark will disappear in a reasonably short length of time under the widely varying temperature and humidity conditions in which tennis is commonly played. That the mark disappears is a requisite in the operation of a satisfactory chemical system. Otherwise, the boundary line areas of a court will become so cluttered with marks that it would become, at best, difficult to determine where a newly arrived ball touched down.
Yet another solution to the problem of accurate line calling involves the use of pressure sensitive switches on tennis court boundary lines to detect the touchdown of balls on those lines. Despite the claims made by the developers of such systems, they have proved incapable of discriminating to an acceptable extent between the touchdown of a ball and the impact made if a player steps on the boundary line, even though relatively complex and costly discrimination circuits have been included in such systems. Furthermore, a player can easily trick a system of this character by striking the boundary line with his racket.
Also, there is a distinct limit on the length which a pressure switch can have and still remain accurate. The numerous spaces between the switches are dead areas. Consequently,there are many areas on a court employing a pressure sensing line calling system which can not detect a ball touching down on the playing surface. One particular and critical circumstance of this character is at the outer edges of a boundary line. A pressure sensitive switch cannot detect with any degree of accuracy whether a ball lands on or just beyond that edge.
Furthermore, a contact which is sufficient for a ball to properly be called in bounds might not generate sufficient force to trigger a pressure sensitive device. In this circumstance, also, a system employing pressure sensitive devices would make the wrong call.
Thus, systems of the foregoing character, such as those disclosed in U.S. Pat. No. 4,365,805 issued Dec. 28, 1982, to Levine for SYSTEM FOR MONITORING TENNIS COURT BOUNDARY LINES and the related, pressure sensor systems discussed in that patent, have not been accepted by the tennis world.
Yet other heretofore proposed systems, such as the infrequently used Cyclops marketed in the United States by Essential Sports, Long Island, New York, rely upon the breaking of an infrared, visible light, radio frequency, or other beam spaced above the tennis court to predict whether a ball will land on the boundary line of a tennis court or other playing surface and therefore touch down in bounds. These systems are often unable to discriminate between beams broken by: (1) an insect flying through the beam, a player's foot or other anatomical member, or a racket or other implement wielded by the player; and (2) the ball or projectile. Because it can be triggered by a player as well as a ball, the Cyclops system can be used only during service (and even then only in singles matches) to detect faults, and a separate operator must be employed to turn the system on and off just before and after a ball is served.
Line calling systems with infrared and pulse duration detectors and other exotica that are purportedly capable of discriminating between a ball and a player's foot have been proposed. However, as far as we know, none of those systems have ever been demonstrated to be practical.
Devices employing light beams and a predictive mode of operation are also difficult to maintain in alignment and notoriously inaccurate, particularly in the cases of looping serves and of balls touching down at high speed and at a low angle. A ball of the just-mentioned character landing as much as two inches behind the outer edge of a boundary line may be called "in" by a Cyclops system.
The possibilities for misalignment are equally objectionable. Play on a court must be halted to correct a misalignment.
Variations in the contour of the court surface can also adversely affect the accuracy of the Cyclops system (or any other line calling system with a detector employing a beam of energy).
In addition, only limited use can be made of such systems for a further reason. The requirement for a transmitter and a detector at opposite ends of each boundary line precludes their use on the boundary line between the add and deuce service courts and elsewhere on a tennis court, depending upon whether a doubles or singles match is in progress.
The Cyclops system has the further disadvantage that only a limited span of coverage is afforded. Thus, this system does not meet the proposed specifications of the U.S. Tennis Association. These require that a line calling system be able to call balls landing as far as two feet out of bounds without human assistance. Multiple beams systems for each boundary line that might conceivably solve this problem. However, they would be prohibitively expensive to manufacture, install, align, and operate, especially in those common circumstances where the playing surface is of the rollup type and is moved from location to location. The software for such a system would be correspondingly complex and cumbersome.
Those systems employing pressure sensing devices are also objectionable for the reasons discussed in the preceding paragraph. No one has as yet disclosed how to make a two foot wide pressure sensor which will accurately detect incoming balls, let alone discriminate between contacts by balls and contacts by a racket, player's foot, etc.
The Cyclops system is also incapable of making net calls. A light beam follows a straight line whereas the top edge of a tennis net describes a catenary curve. It is contact with the net that counts, not contact with an imaginary straight line which coincides with the top of the net only at the ends of the net. Thus, a light beam can not make let calls with any degree of accuracy.
Strain gauges and audio transceiver systems have also been employed in net calling systems. These, too, are unsatisfactory. For example, a light breeze or impact by a tennis racket can trigger any strain gauge system sensitive enough to detect ball-with-net contact. And partisan fans quickly learned that they could defeat an audio let detecting system with a loud hand clap.
Yet another touchdown sensing system heretofore proposed is that disclosed in U.S. Pat. No. 3,774,194 issued Nov. 20, 1973, to Jokay et al. for GAME COURT BOUNDARY INDICATOR SYSTEM. In the Jokay et al. system, a specially designed ball: "has an effect on a secondary antenna system" buried beneath the playing surface. This change in the antenna signal is detected and utilized to provide an indication that the ball touched down on a boundary line.
Tennis balls with the circuitry required to effect detectable changes in the signal received by a buried antenna system are impractical and would probably be prohibitively expensive to manufacture. Also, the buried antenna systems would probably be equally expensive to install. Furthermore, the use of radio frequency signals would lead to obvious problems in the areas of tuning, sensitivity, and RF noise
A line calling system with similar objections uses the detection of a magnetic flux change to ascertain whether a ball lands in or out of bounds. Aside from being inaccurate, such a system would be expensive to install and maintain; and it would probably not even be capable of determining if a ball came from across a net or simply bounced back onto the playing surface from an out of bounds location closely adjacent the boundary line.
Yet other systems for eliminating fallible human determinations of whether a tennis all has touched down in or out of bounds are disclosed in above-discussed Van Auken '911 patent. Those systems include a network of parallel, spaced apart, exposed electrical leads extending along each of the tennis court boundary lines and a companion network of similarly related leads lying in the out of bounds area immediately adjacent each boundary line. Alternate leads of each network are connected to an electrical power source, and the remaining leads are grounded.
A conductive tennis ball is employed. If that ball lands on a boundary line when it touches down and contacts two adjacent leads of the boundary line network, current will flow in a circuit containing those two leads. This current is a signal indicating that the ball landed in bounds.
A ball touching down in the out of bounds area immediately adjacent a boundary line may similarly bridge two leads or conductors of the out of bounds network located in that area. This provides an indication that the ball touched down out of bounds.
While a vast improvement over those other line calling systems discussed above, the system disclosed in the Van Auken '911 patent nevertheless has its disadvantages and drawbacks.
One is that it is maybe less accurate than desirable because no effort is made to locate conductors precisely on the outer edges of the boundary lines. As a consequence, a ball touching down and having a footprint initially in an out of bounds area but then expanded to the outer edge of the boundary line may not contact a boundary line-associated conductor. As a consequence, such a ball may be determined by the patented system to be out of bounds even though it should have been called "in" in accord with the official rules of tennis.
In addition, those techniques for providing conductor networks which Van Auken '911 discloses have been found to be less than entirely satisfactory. Conductive threads as disclosed in that patent (made by twisting together silver plated Nylon fibers) were found to be less durable than is desirable, and the electrical characteristics of those threads also left something to be desired.
Extruded conductors fitted into grooves in rigid playing surface panels as described in Van Auken '911 proved even less satisfactory because of the time and expense involved in cutting the grooves and in leveling the court with the precision required for the playing surface to lie flat. In addition, the heat of the sun caused the ends of the rigid panels to curl upwardly resulting in an unacceptable, uneven playing surface.
Another significant disadvantage of the line calling systems disclosed in Van Auken '911 is that the players or official (s) can not selectively activate or deactivate the conductor networks associated with particular tennis court boundary lines. This function is a requisite in a practical system as different boundaries are employed: (a) in singles and doubles play, and (b) (i) during service, and (ii) any subsequent volleying.
Related disadvantages of the Van Auken '911 system are that no provision is made for configuring that system to: (1) provide the different outputs that are appropriate to attended and unattended play, or (2) select different combinations of visual and audio signals, or (3) make both in bounds and out of bounds calls or out of bounds calls only.
Other touchdown sensing systems employing electrical conductors and a conductive ball and having drawbacks like those discussed above in conjunction with the Van Auken '911 patent are disclosed in the following U.S. Pats. 3,883,860 issued May 13, 1975, to Von Kohorn for ELECTRICAL INDICATOR SYSTEM FOR BALL GAMES; 4,071,242 issued Jan. 31, 1978, to Supran for ELECTRICALLY CONDUCTIVE TENNIS BALL; and 4,299,384; 4,433,840; and 4,664,378, all issued to John A. Van Auken and respectively entitled ELECTRICALLY CONDUCTIVE GAME BALL, ELECTRICALLY CONDUCTIVE GAME BALL, and ELECTRICALLY CONDUCTIVE TENNIS BALL.